1. The Field of the Invention
This invention relates to infrared (IR) instruments and, more particularly, to novel systems and methods for long-term, in-flight calibration of IR instruments.
2. The Background Art
IR instruments must periodically be calibrated in order to provide data of desired or required accuracy. However, the accuracy and stability of temperature sensors used in such calibration procedures can be affected by numerous factors. For example, measurement systems using on-board blackbodies as reference points are subject to temperature sensor drift.
Factors affecting the performance of a temperature sensor may include sensor configuration and type. Other factors may include the thermal environment in which a temperature sensor operates, shock or vibration experienced by a temperature sensor, the nature of the thermal contact between a temperature sensor and a source or blackbody, strain in connecting wires, self heating, and age of a temperature sensor. Any of these factors may cause drift and necessitate recalibration.
In the past, to better understand the behavior of a temperature sensor, testing has been performed in the environment in which the temperature sensor will operate. Multiple cycles over the range of operation have been used to establish drift rates and the noise characteristic for particular temperature sensors. Accordingly, manufacturers can publish average drift rates for their temperature sensors. These drift rates are, at best, typically about 25 mK/year.
Manufacturers often recommend that temperature sensors be re-calibrated yearly. However, calibration may be performed more or less frequently based on performance, requirements, and environment. IR instruments measuring climate conditions typically have expected lifetimes of seven to ten years in orbit (i.e., sometimes called space, outer space, flight, or a micro-gravity environment). Accordingly, re-calibration of IR instruments is a necessity.
Current methods for compensating for drift include cross correlation of sensor data with that of other instruments viewing the same Earth scene. This Simultaneous Nadir Overpass (SNO) method allows offsets between instruments to be corrected, but does not allow absolute calibration. What is needed is a system providing a long-term, in-flight, calibration system, particularly one that may be relied upon as consistent and absolute.